Receivers for cellular radio equipment are subject to large variations in the radio environment as a result of the typical No-Line-Of-Sight (NLOS) communication, the varying distance between user equipment (UE) and a base station, as well as the presence of other UEs and base stations operating in the same band. These variations manifest themselves as varying strengths and frequencies of interfering signals and varying strengths of desired signals.
As the evolution of cellular standards strive towards ever increasing throughput by using higher order modulation, extending bandwidth, using multi-antenna techniques, using new coding schemes, etc., power consumption by radio modems of UEs may increase accordingly. This, obviously, may be particularly harmful for the UE that has a limited battery capacity. Therefore, making transceivers, and the receiver part in particular, more flexible to adapt performance dynamically with respect to the varying radio environment may be a key factor for significantly reducing power consumption. To dynamically adapt performance of the transceiver, radio environment scanning may be needed. Currently, UE designs tend to set performance to handle a worst-case scenario, which may be taxing on a battery of the UE.
Cognitive radio is a concept that is applicable to wireless communication. Cognitive radio, as known in the art, involves a network and/or a wireless node (such as a UE) changing its transmission and/or reception parameters to communicate efficiently so as to optimize throughput (or other Quality of Service (QoS) metrics) and avoid interference with other devices. To change transmission and/or reception parameters, the network and/or wireless node may perform active scanning of the radio environment (known as having “radio-environment awareness”).
While the above describes more general views on the need for radio environment scanners, the example below is a rather specific, but still important, example demonstrating one particular issue.
A common problem for Frequency-Division Duplex (FDD) radio transceiver equipment, with concurrent reception and transmission, is cross-modulation products generated in the receiver between a transmission (TX) signal and an interfering signal. As illustrated in FIGS. 1A and 1B, a transmission signal, from a transmitter (TX) in a particular UE, may leak through a duplex filter to an input of a receiver (RX). As shown in FIG. 1A, an interfering signal may be generated by an external interfering transmitter.
Alternatively, in those situation where the UE includes a second radio transceiver (e.g., a Bluetooth or Wide Local Area Network (WLAN) transceiver), as shown in FIG. 1B, the signal transmitted by the second radio transceiver (e.g., an interfering transceiver) may be captured by the UE antenna of the first radio transceiver (e.g., an interfered transceiver) and leaked through the duplex filter into the radio receiver of the first radio transceiver. When both the TX and interfering signals are strong, the cross-products resulting from limited linearity in the RX front-end of the receiver may fall into a desired RX band, thereby reducing a signal-to-noise ratio (SNR) of that signal. However, this situation may only happen if the interfering signal is either located at the other side of the TX signal (with respect to the RX signal) at a distance equal to a duplex distance (fd) from the TX signal or exactly between the TX and RX signals, as illustrated in FIGS. 1C and 1D, respectively. In each of these situations, as illustrated in FIGS. 1C and 1D, the TX signal and interfering signal are positioned to make a cross-product appear in the RX band.
Yet another situation is shown in FIG. 1E. In FIG. 1E, the transmitter, which causes the interfering signal, may be so close to the RX signal that spectral regrowth of the signal in the radio receiver, due to the interfering signal, falls into the desired RX band. The interfering signal could either be the TX signal, when the duplex distance is very small, or an interfering signal.
The situations described above typically dictate the requirements for the linearity of the receiver front-end that has to be designed to cope with the worst-case scenario in this respect. Having a receiver design with fixed parameters designed to cope with the worst-case scenarios as discussed above will have a linearity performance and power consumption that are unnecessarily high for most scenarios. Radio environment scanners typically have to measure signals at several frequencies or at a range of frequencies. Thus, conventional radio environment scanners are associated with a dedicated radio frequency (RF) local oscillator (LO) synthesizer for down-conversion that is stepped over the frequency range of interest.